Definition
A symbol used on engine instruments and in technical documentation to represent the rotational speed of the low-pressure compressor (and the low-pressure turbine that drives it) in a dual-spool gas turbine engine. N1 is typically displayed as a percentage of the maximum rated rpm rather than as actual rpm.
Plain English
N1 is the speed of the inner spinning section of a turbine engine that has two independent rotating sections. It tells you how fast the front fan and low-pressure compressor are turning, shown as a percentage of the engine's maximum allowed speed.
Context Anchor
Seen in turbine engine maintenance, engine instruments, and powerplant discussions about compressor speed.
Derivation
The 'N' is the standard engineering symbol for rotational speed. The '1' refers to the first (low-pressure) spool. In a dual-spool engine, the spools are numbered from the front of the engine: N1 is the low-pressure spool, N2 is the high-pressure spool. A third spool, where present, is N3.
Why Pilots Care
Pilots use N1 to confirm engine thrust output and detect performance issues before they affect flight safety.
Intuition Check
Do not read NN1 as an engine model name or a part number. Here it is a speed symbol for one rotating section of a turbine engine.
Example Sentence 1
After starter cutout, the crew monitored N1 as it stabilized at idle before advancing the thrust levers.
Example Sentence 2
During the after-start checklist the crew verified both engines showed normal N1 indications.